1. Field of the Invention
The present invention relates to a valve train apparatus for 4 stroke-cycle internal combustion engine.
2. Description of Background Art
Recently the 4 stroke-cycle internal combustion engine has been provided with a valve train apparatus in which output of the engine can be controlled by controlling an amount of intake air with carrying out continuously variable control of an amount of lift of an intake valve and the valve timing in accordance with running condition of a vehicle as well as high performance during a high loading condition and improvement of fuel consumption with reducing a pumping loss during a low loading condition can be simultaneously achieved.
In a conventional valve train apparatus for the 4 stroke-cycle internal combustion engine, the continuous variation of lift of intake valve is achieved by controlling an intake valve with controlling movement of a rocker arm, an intermediate lever and an eccentric cam shaft independent from the intake cam shaft. Such conventional valve train apparatus is described in, for example, the Germany magazine MOTORTECHNISCHE ZEITSCHRI FT (herein “MTZ”), Vieweg Verlag/GWV Fachverlage GmbH, 11/2004, pages 868, 876 and 878, and the Germany public magazine BMW Aftersales (herein “BMW”), BMW Corporation, Nr. 75, page 18.
According to the conventional invention described in the above-noted magazines, the continuous variation of the lift of intake valve is achieved by controlling an intake valve with controlling movement of a rocker arm, an intermediate lever and an eccentric cam shaft independent from the intake cam shaft. The intake cam shaft, the exhaust cam shaft and the eccentric cam shaft are supported at heights different from each other and secured via parting surfaces of different directions and screws. The parting surface of the exhaust cam shaft is formed by an upper surface parallel to a bottom surface of cylinder head, the parting surface of the intake cam shaft is formed by a surface at intake port side normal to the bottom surface of cylinder head, the parting surface of the eccentric cam shaft is formed by a surface at exhaust port side normal to the bottom surface of cylinder head, and a head cover mounting surface includes a parting surface of the exhaust cam shaft parallel to the bottom surface of cylinder head, upper surfaces of different heights and four machined surfaces of three different directions and secured by screws. Similarly each bearing cap is also formed by three separate parts and fastened by screws from three different directions. In addition, since the eccentric cam shaft is arranged at a highest position, a motor for controlling the eccentric cam shaft must be positioned at a very high position.
As shown in the magazine MTZ, the intermediate lever of the conventional invention is a straight rod shaped configuration and has a roller contacting an eccentric shaft of the intermediate lever, a sliding portion contacting a roller of intake rocker arm of the intermediate lever, and a roller contacting a intake cam shaft of the intermediate lever which are all arranged on a straight line. In addition, the roller contacting the eccentric cam shaft and the sliding portion contacting the roller of the intake rocker arm are arranged vertically opposite with respect to the central roller contacting the intake cam shaft.
In the valve train apparatus for the 4 stroke-cycle internal combustion engine of the conventional invention, there are problems of an increase of an overall height of a cylinder head, increase of machining surfaces for supporting the intake cam shaft, exhaust cam shaft and eccentric cam shaft, increase of machining directions of fastening screws, difficulties and complication of assembly, and substantial increase of manufacturing costs due to an increase of the number of structural parts. In addition, since the intermediate lever is made as a straight rod shaped member having heavy weight and low rigidity, it is difficult to reduce its length. This also causes the increase of the overall height of the cylinder head.